The present invention relates to stabilized power supplies called chopped supplies.
A chopped power supply operates in the following way: a primary transfer winding receives a current which is for example delivered by a rectifier bridge receiving the power of the AC mains. The current in the transformer is chopped by a switch (for example a power transistor) placed in series with the primary winding.
A circuit for controlling the transistor produces periodic square waves for enabling the transistor. A current is allowed to pass for the duration of the square waves; outside the square wave, the current cannot pass.
On one (or more) secondary windings of the transformer, an AC voltage is collected. This is rectified and filtered so as to obtain a DC voltage which is the output DC voltage of the chopped power supply.
For stabilizing the value of this DC voltage, the cyclic period conduction ratio of the switch is adjusted, that is to say the ratio between the duration of conduction and the duration of non conduction in a chopping period.
In chopped power supply architecture proposed by the applicant and shown in FIG. 1, two integrated circuits are used. One of the circuits CI1, serves for controlling the base of a power transistor T.sub.p for applying thereto periodic enabling and disabling control signals. The base control circuit CI1 is placed on the primary winding side (EP) of the transformer (TA) for reasons which will be better understood in the rest of the description. The other integrated circuit, regulation circuit CI2, is on the contrary placed on the secondary side (winding ES1) and it serves for examining the output voltage Vs of the power supply for forming regulation signals which it transmits to the first integrated circuit through a small transformer TX. The first integrated circuit CI1 uses these regulation signals for modifying the cyclic conduction ratio of the switching transistor T.sub.p and thus regulating the output voltage Vs of the power supply.
We will come back further on in more detail to the circuit of FIG. 1.
Numerous problems arise during the design of a chopped power supply, and here we will consider more particularly the problems of starting up the supply and the problems of safety in the case of over voltages or over currents at different points in the circuit.
The first problem which is met with is that of starting up the power supply: at switch on, the regulation circuit CI2 will tend to cause the base control circuit CI1 to generate maximum cyclic ratio square waves until the power supply has reached its nominal output voltage. This is all the more harmful since there is a high current drain on the side of the secondary windings which are connected to initially discharged filtering capacitors. There is a risk of destruction of the power transistor through an overcurrent during the start up phase.
Circuits for gradual start up have already been proposed which limit the duration of the enabling square waves during a start up phase, on switching on the device; the U.S. Pat. No. 3,959,714 describes such a circuit in which charging of a capacitor from switch-on defines initially short square waves of gradually increasing duration until these square waves reach the duration which the regulation circuit normally assigns to them. The short square waves have priority; but, since they become gradually longer during the start up phase, they cease to have priority after a certain time; this time is defined by the charging time constant of the capacitor.
Another problem to be reckoned with is the risk of accidental over-currents, or sometimes over-voltages which may occur in the circuit. These overcurrents and over-voltages may be very detrimental and often result in the destruction of a power transistor if nothing is done to eliminate them. In particular, a short circuit at the output of the stabilized power supply rapidly destroys the power transistor. If this short circuit occurs on switching-on of the supply, it is not the gradual start up system with short and progressively increasing square waves which can efficiently accomodate the over-currents which result from this short circuit.
Finally, another problem particularly important in an architecture such as the one shown in FIG. 1, is the risk of disappearance of the regulation signal which should be emitted by the regulation circuit CI2 and received by the base control circuit CI1: these signals determine not only the width of the square waves enabling the power transistor but also their periodicity; in other words, they serve for establishing the chopping frequency, possibly synchronized from a signal produced on the secondary side of the transformer. The appearance of these signals causes a particular disturbance which must be taken into account.
Furthermore, the architecture shown in FIG. 1, in which the secondary circuits have been voluntarily separated galvanically from the primary circuits, is such that the base control circuit may operate rapidly after switch-on, as will be explained further on, whereas the regulation circuit CI2 can only operate if the chopped power supply is operating; consequently, at the beginning, the base control circuit CI1 does not receive any regulation signals and this difficulty must be taken into account.